Quantum Mechanics and Why You Should Care

@tine, of course I’d love to

The two questions you seem to be asking are:
1. How can we reason about the rest of the universe simply from our experience here on Earth?
2. What empirical proof is there that Quantum theory is correct?

Here are your answers my sir:
1. We can’t. There’s this belief that physicists have, which I’m going to call space equality, which says that every point in space is subject to the same laws as every other point in space. Now, this is an assumption. There is no logical basis for this fact. It’s similar to the idea of induction, the idea that if something has happened n times (where n is very large), it will happen n+1 times. Note this is separate from mathematical induction, which is deductive, because induction as method of argument discovery about the world has no logical basis. It is inductive; it doesn’t prove things for certain. But let’s examine whether we have adequately explored enough space such that n is large enough to say we have a strong argument…I think everyone would agree the answer is no. We orbit the sun – we have sent very few things to other places – the universe is huge – and therefore we have explored almost 0% of it. BUT, there is one method we have for collecting evidence about the laws of the universe at other points in space. We can analyze phenomenon on Earth and apply them to things we see elsewhere in space. If our predictions line up with reality, then it’s reasonable to guess that the rest of space obeys the same laws of physics. But we still don’t escape the problem of induction, and thus the answer to your question is that we don’t know that the universe is uniform. We think it’s so, but we’re not sure. But that’s all science, isn’t it? To say something absolutely is the thing of religion and non science. Here’s some Richard Feynman for you:
“We absolutely must leave room for doubt or there is no progress and there is no learning. There is no learning without having to pose a question. And a question requires doubt. People search for certainty. But there is no certainty. People are terrified – how can you live and not know? It is not odd at all. You only think you know, as a matter of fact. And most of your actions are based on incomplete knowledge and you really don’t know what it is all about, or what the purpose of the world is, or know a great deal of other things. It is possible to live and not know.”

As a last thought, there actually has been discussion that the laws of physics can change with time and space – the idea being that the way the laws change can be described by some other more fundamental law. This isn’t a popular view in physics, but that doesn’t mean it’s wrong – it’s entirely plausible.

2. As to your latter question – how do we know quantum mechanics is correct – once again the answer is that we don’t. This one should be slightly more satisfying than the last though.

First, the point must be made that more than any other scientific model, quantum mechanics is very much a match-with-observation theory. There are many physicists out there that think that although it predicts many things correctly, it may not be the fundamental underlying process; it matches observation, but how do we know there isn’t something else going on we aren’t seeing? In fact I’d claim it’s the single-most doubted theory in all of science given the mountain of evidence we have supporting it.

Which leads us to the evidence discussion. There’s a mountain of evidence behind quantum theory but it all follows the same idea as the following example:
Once there was this guy called Schrodinger. People challenged him to find an equation for this new idea he had. He went away on vacation with his wife and came back with some equation. Google it if you want (Schrodinger wave equation). Nobody knew how he came up with it, and he never told. It has imaginary numbers in it – something that people thought was insane at the time. All in all, it’s a pretty sketchy equation. But lo and behold, they went and solved it (oversimplification) and got predictions about where electrons should reside in atoms (for physicists out there, you know I’m kind of forcing this one but spherical harmonics can be derived from the schrodinger equation as well). But remember, it’s quantum mechanics, so you don’t get a definite answer. Rather, you get probabilities and clouds. In order to compare with empirical evidence, the best you can do is record how frequently you get a result and see if it makes sense in the context of your cloud. If you’ve studied statistics, it’s like performing a test to see if you are pulling from the distribution you think you are pulling from. The result was extremely strong agreement between quantum theory and reality. So do we think quantum theory is right? Meh, not necessarily. We know the universe might be doing something else underneath the hood, but whatever it’s doing, it mimics this other thing we found extremely closely, so why not go with our best guess, right? That’s science, just our best guess. (As for other evidence, there are mountains of it, but it all follows the same scheme; you pull results from a distribution and do statistical tests to see if things are in fact what you thought they were)

BRINGING IT BACK – your original question has to do with the many-worlds-interpretation of quantum mechanics. How can we talk about the state of the universe as a cloud if we can only experience one part of the cloud? Well, we came up with a mathematical idea. It fit with reality so we kept it. The consequences of the theory predicted something we cannot possibly test. So do you believe it? Don’t believe that it’s true – believe that it’s plausible. Like Feynman said, you can live and not know. It’s possible to live and not know.